The present invention relates to catalyst recovery from a light FCC-type effluent, and also to regeneration of the recovered catalyst.
It has been proposed to produce light olefins such as ethylene and propylene from mixtures of heavier paraffin and olefins using a fluid catalytic cracking (FCC) system with the reaction conditions described, for example, in U.S. Pat. No. 5,043,522 to Leyshon et al.; U.S. Pat. No. 5,171,921 to Gaffney et al.; and U.S. Pat. No. 6,118,035 Fung et al. In this system, particulated catalyst and feedstock enter a reactor under specific reaction conditions. The reactor effluent is processed in a series of cyclone separators, usually housed in a vessel, that separate most of the catalyst from the effluent to be regenerated for recycle to a regenerator and then to the reactor, in a manner similar to conventional refinery FCC operations. The catalyst-lean hot effluent gases from the cyclones are then cooled and separated by fractional distillation, for example, into the product constituents.
There are, however, some significant differences between the light olefin FCC process and conventional refinery FCC operations. Conventional FCC processes produce an effluent that has significant quantities of heavier hydrocarbons that are condensed in a quench tower. There is also a minor amount of residual catalyst entrained in the effluent, that is not removed by the cyclones, which is collected with the heavier hydrocarbons condensed in the quench tower to form slurry oil. Slurry oil from the quench tower is often difficult to process and/or dispose of; frequently it is burned as a fuel oil. In the light olefin FCC process, there are only very minor quantities of heavier hydrocarbons in the effluent gas, i.e. a relatively high ratio of catalyst to fuel oil, so the removal of the catalyst fines becomes problematic because there is very little heavy oil recovered and any “slurry oil” would have a much higher catalyst loading than in the case of the conventional refinery FCC process.
Another issue in the light olefin FCC process is the regeneration of the catalyst recovered from the riser effluent by the cyclones. In the conventional refinery FCC unit, significant quantities of coke are formed in the riser and deposit on the catalyst particles. In the regenerator, this coke can be used as a fuel source for combustion with oxygen in the regenerator vessel to supply the heat needed to heat-balance the unit. Frequently, the regenerator may need to be cooled to prevent the catalyst from getting too hot, particularly when the feedstock deposits a lot of carbon on the catalyst. On the other hand, the prior art light olefin FCC process generally has insufficient coke deposition in the light olefin FCC process to support catalyst regeneration and the heat of reaction.
In a conventional gasoline FCC process, it has been suggested that supplemental fuel such as fuel gas or fuel oil (torch oil) can be introduced into the regenerator to achieve the temperatures required for catalyst regeneration and the heat of reaction during non-steady state operations, for example, when starting-up the unit, to achieve an adequate regenerator temperature. However, as far as applicant is aware, there are no adequate systems known for introducing fuel into the dense phase bed of a FCC regenerator processing low-carbon catalyst, for continuous operation.
Further, there is a need for a light olefin FCC process and system capable of processing a light feedstock that conventionally yields inadequate coke formation, yet improved somehow to achieve the heat of reaction required in the reactor.